Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.).
One of the most useful semiconductor structures is the p-n junction. The basic building block of many electronic and electrical devices, the p-n junction tends to conduct an electric current in one direction and blocks it in the other, and tends to generate an electric field. This last property is useful for charge extraction applications such as solar cells.
In a typical solar cell, absorbed light will generally create an electron-hole pair. Electrons on the p-type side of the junction within the electric field (or built-in potential) may then be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
Most solar cells are generally formed on a silicon wafer doped with a first dopant (commonly boron) forming an absorber region, upon which a second counter dopant (commonly phosphorous) is diffused forming the emitter region, in order to complete the p-n junction. After the addition of passivation and antireflection coatings, metal contacts (fingers and busbar on the emitter, and pads on the back of the absorber) may be added in order to extract generated charge. Emitter dopant concentration, in particular, must be optimized for both carrier collection and for contact with the metal electrodes.
In general, a low concentration of dopant atoms within an emitter region will result in both low recombination (thus higher solar cell efficiencies), and poor electrical contact to metal electrodes. Conversely, a high concentration of dopant atoms will result in both high recombination (reducing solar cell efficiency), and low resistance ohmic contacts to metal electrodes. Often, in order to reduce manufacturing costs, a single dopant diffusion is generally used to form an emitter, with a doping concentration selected as a compromise between recombination and ohmic contact. Consequently, potential solar cell efficiency (the percentage of sunlight that is converted to electricity) is reduced.
One solution is the use of a dual-doped or selective-emitter. A selective emitter uses a first lightly doped region optimized for low recombination, and a second heavily doped region (of the same dopant type) optimized for low resistance ohmic metal contact. However, a selective-emitter configuration may be difficult to achieve in a one-step diffusion process and may involve several masking steps, consequently increasing manufacturing costs. In addition, since there are generally no visual boundaries between high doped and low doped emitter regions, the alignment of a metal contact onto a previously deposited highly doped region may be difficult.
Like the emitter region, the deposition of a BSF (back surface field) may also be problematic. A BSF is generally a region located at the rear of a solar cell which tends to repel minority carriers in the absorber region from high recombination zones at the rear surface and metallized regions of the wafer. In general a BSF may be formed using dopants of the same type as those used in the absorber region, in this case the concentration of dopant atoms in the BSF is selected to be higher than that used to dope the absorber region, thus creating a potential barrier between the bulk of the wafer and the rear surface.
In addition, in a typical solar cell structure the BSF is generally formed using aluminum (or other deposited materials) which is generally first screen printed onto the back of a solar cell and then co-fired in a belt furnace along with the front side metal contacts (commonly formed from screen printed silver paste). Typically, silicon atoms in the wafer tend to diffuse in the aluminum and subsequently recrystallize, incorporating aluminum atoms into the silicon crystal. However, although relatively easy to manufacture, the thermal expansion coefficient of aluminum (about 24 μm/m° C.) is much greater than silicon (about 3 μm/m° C.). Consequently, wafer bowing tends to occur. And while some reduction in carrier recombination is achieved with the screen printed A1 BSF, there is still significant recombination occurring at the rear which tends to reduce solar cell efficiency.
Alternatively, the rear surface may be passivated by the diffusion of dopant atoms of the opposite type (counter dopant) to those used in the absorber region. In this case a floating junction is established at the rear side of the substrate which has been shown to also provide effective passivation. A second diffused region must generally be used to provide ohmic contact to the absorber region of the solar cell.
Finally it is possible to use a localized heavily doped region to form ohmic contacts only over a small region of the rear surface and to reduce recombination in other regions using surface passivation layers (e.g. SiNx, TiO2, SiO2). In this case the surface layers both reduce the recombination sites at the surface of the silicon as well as providing fixed charge that repels minority carriers from the surface. Similar to the formation of the high efficiency emitter (e.g. selective emitter), formation of an effective BSF often involves many processing steps and is therefore often too costly for manufacturing.
In view of the foregoing, there is a desire to provide an optimized method of forming multi-doped junctions on a substrate.